1. Field of the Invention
The present invention generally relates to an optical disk device, and more particularly, to an optical disk device causing a light beam to be projected intermittently with different powers.
2. Description of the Related Art
FIG. 1 is a block diagram of an optical disk device. FIG. 2 is an illustration for explaining a structure of an optical disk.
An optical disk device 100 shown in FIG. 1 is a DVD-R drive, for example; a DVD-R disk 40 is mounted thereon so that the optical disk device 100 records/reproduces information to/from the DVD-R disk 40.
On the DVD-R disk 40, as shown in FIG. 2, a wobble 40b is formed along a track 40a to/from which information is recorded/reproduced. The wobble 40b is so formed as to undulate in radial directions of the disk at a predetermined cycle. By reproducing the wobble 40b, a disk revolution control signal, a pre-pit detection gate signal and so forth are obtained.
Additionally, on the DVD-R disk (or a DVD-RW disk), an address on the disk is recorded as LPPs (land pre-pits) pre-pitted on the land (the track) 40a. 
The optical disk device 100 comprises an optical system 41, a spindle motor 42, a sled motor 43, a laser driver 44, a front monitor 45, an ALPC (Auto Laser Power Control) circuit 46, a recording compensation circuit 47, a wobble signal processing unit 48, an RF amplifier 49, a focus/tracking servo circuit 50, a feed servo circuit 51, a spindle servo circuit 52, a DVD encode/decode circuit 53, RAMs 56 and 58, an interface/buffer controller 59, and a CPU 60; the optical disk device 100 records/reproduces information according to a command from a host computer 61.
The spindle motor 42 is driven by the spindle servo circuit 52 so as to cause the disk 40 to revolve at a predetermined revolving speed. The optical system 41 is positioned opposite the disk 40. The optical system 41 includes a laser diode and a quadripartite photodetector. The optical system 41 projects a laser light on the disk 40 so as to record information on the disk 40, and also outputs a reproduction signal corresponding to recorded information according to a light reflected from the disk 40. The quadripartite photodetector is divided in four zones of A, B, C and D, for example. As shown in FIG. 2, the zone A detects a portion of the reflected light of a laser beam LB, the portion being located at an outer side, i.e., at an arrow-A1 side, and at a scanning-direction side of the laser beam LB, i.e., at an arrow-B1 side. As shown in FIG. 2, the zone B detects a portion of the reflected light of the laser beam LB, the portion being located at an inner side, i.e., at an arrow-A2 side, and at the scanning-direction side of the laser beam LB, i.e., at the arrow-B1 side.
As shown in FIG. 2, the zone C detects a portion of the reflected light of the laser beam LB, the portion being located at the inner side, i.e., at the arrow-A2 side, and at a non-scanning-direction side of the laser beam LB, i.e., at an arrow-B2 side. As shown in FIG. 2, the zone D detects a portion of the reflected light of the laser beam LB, the portion being located at the outer side, i.e., at the arrow-A1 side, and at the non-scanning-direction side of the laser beam LB, i.e., at the arrow-B2 side.
The optical system 41 is controlled by the sled motor 43 and the focus/tracking servo circuit 50 in positioning the light beam LB projected on the disk.
The above-mentioned sled motor 43 is driven and controlled by the feed servo circuit 51 so as to cause a carriage composing the optical system 41 to move in the radial directions of the disk 40. The focus/tracking servo circuit 50 drives and controls a focus and tracking actuator (not shown in the figure) of the optical system 41 so as to perform a focus/tracking control.
The reproduction signal reproduced by the optical system 41 is supplied to the RF amplifier 49. The RF amplifier 49 amplifies the reproduction signal. A primary signal of the reproduction signal is supplied to the DVD encode/decode circuit 53, and is decoded thereby. Additionally, various servo signals are extracted and output to each of the servo circuits.
The RAM 56 is used as a working storage for processes in the DVD encode/decode circuit 53. The interface/buffer controller 59 exchanges data with the host computer 61, and controls a data buffer. The RAM 58 is used as a working storage for the interface/buffer controller 59.
The CPU 60 controls the optical disk device 100 as a whole according to commands from the host computer 61.
On an optical disk, such as the DVD-R disk, the wobble is formed beforehand along the track that is to be formed so as to record information. By detecting this wobble, a wobble signal is reproduced. Additionally, on the disk, the pre-pits are formed on the land. By reproducing these pre-pits, information, such as an address representing a disk position is obtained. In this course, in order to obtain accurate information, such as an address, a pre-pit signal needs to be converted accurately into digital data.
Besides, the LPP needs to be read out accurately also before recording, during recording, and after recording.
At this point, a description will be given of a method of detecting the pre-pits.
FIG. 3 is a diagram for explaining a conventional pre-pit detecting method.
To detect the pre-pits, [(Sa+Sd)−(Sb+Sc)] is obtained, first, assuming that a detection signal in the zone A shown in FIG. 2 is Sa, that a detection signal in the zone B is Sb, that a detection signal in the zone C is Sc, and that a detection signal in the zone D is Sd. In FIG. 3, a solid line represents a waveform of (Sa+Sd), and a dashed line represents a waveform of (Sb+Sc).
Next, by comparing a waveform of [(Sa+Sd)−(Sb+Sc)] with a predetermined level L11 as a threshold value, the LPP is detected. However, during recording, a power of the laser beam LB fluctuates in a pulse form according to information being recorded. This fluctuation becomes noises which impede the detection of the LPP.
FIG. 4 and FIG. 5 illustrate how the fluctuation of the power of the laser beam LB impedes the detection of the LPP. In FIG. 4, a solid line represents the waveform of (Sa+Sd), and a dashed line represents the waveform of (Sb+Sc). FIG. 5 represents the waveform of [(Sa+Sd)−(Sb+Sc)].
In FIG. 4 and FIG. 5, the power of the laser beam is at a write power level at times t1 and t2, and the power of the laser beam is at a read power level at a time t3.
At the times t1 and t2, since the pre-pits are detected with the write power level, the pre-pits are clearly distinguished from surroundings; accordingly, signal levels at the pre-pits are prominent. At the time t3, since the laser beam at the read power level is projected on the pre-pit, a detection signal level becomes smaller than the detection signal levels that are detected when the laser beam at the write power level is projected on the pre-pits. Accordingly, there has been a risk that the detection signal of the pre-pit at the time t3 may become undetectable being buried in the levels of wobble signals detected with the write power level.
As a solution therefor, there has been proposed a technology described in Japanese Laid-Open Patent Application No. 10-283638. In the technology described in Japanese Laid-Open Patent Application No. 10-283638, the LPP is detected at a gate timing of a sample-and-hold circuit, etc.
However, since the technology described in Japanese Laid-Open Patent Application No. 10-283638 necessitates the gate timing of the sample-and-hold circuit, etc., a control therefor is complicated. Additionally, there is a problem that noises occur due to the sampling-and-holding. Further, there is a problem that the sampling-and-holding becomes difficult to perform as a recording speed becomes higher.